The present invention relates generally to a system for detecting the presence of a contaminating fluid. More particularly, the present invention relates to a system for detecting the presence of hydrocarbons (such as petroleum products) in water or the presence of water in hydrocarbons. It is often necessary or desirable to detect the contamination of one fluid by another. Examples are hydrocarbons leaking into a line carrying boiler feed water, and water leaking into a petroleum plant feed stream. In either case contamination could cause serious damage to the equipment and loss of product. Also, in many instances the contamination of one fluid by another results in pollution of the environment.